4. Anthropogenic pressures

IMHEN. When the GWL reaches 1.5°C, the dynamical experiments show rainfall increases in the range of 11.6–17.5% in the Mekong Delta, while the BCSD results shows slight rainfall decreases of less than 3% over most of the region. When the GWL rises to 2.0°C, the IMHEN experiments show a slightly larger increase in rainfall in the range of 13.6–18.5%; while the BCSD shows an increase of 3–5% and areas with increasing rainfall are projected to ex-

4.1 Upstream dams and sand mining drive sediment starvation

The flowing water and moving sediments of the Mekong River are fundamental to the existence of the Mekong Delta. Basin-wide alterations of natural water and sediment dynamics have had the biggest impact on the ecological and geophysical function of rivers and their deltas worldwide [ Grill et al., 2019 ]. Due to upstream impoundments and downstream interventions (e.g., channel fixing with levees, dyking populated areas), fluvial sediment supply to world deltas has decreased by 30% [ Vörösmarty et al., 2003; Syvitski et al., 2009; Syvitski and Kettner, 2011; Besset et al., 2019; Best, 2019 ], and 40–50 109 T/yr of global demand for sand [ UNEP, 2019 ] is incising rivers and estuarine systems [ Bravard et al., 2013; Brunier et al., 2014; Best, 2019 ]. Once one of the last uninterrupted rivers in the world, over the past three decades, the Mekong River has rapidly joined that trend, and sediment starvation has already caused irreparable damage along the river and pand. For the higher GWLs of 3.0°C and 4.0°C, the increases in average rainfall obtained from the dynamical downscaling experiments in the Mekong Delta are 22.2% and 20.8%, respectively. Meanwhile, the BCSD results show respective increases of only 3.7% and 7.9%. At the 4.0°C GWL, both the statistical and dynamical experiments project increasing rainfall trends across the entire VMD.

within the VMD. The estimate [ Milliman and Farnsworth, 2011 ] of total sediment transported by the pristine Mekong river at the entrance of the delta was ~160 Mt yr1, but recent estimates show 40–90% reduction in fluvial sediment supply [ Kummu and Varis, 2007; Walling, 2009; Kummu et al., 2010; Koehnken, 2014; Lu et al., 2014; Fan et al., 2015; Manh et al., 2015; Darby et al., 2016; Dang et al., 2018 ]. The effects of sediment trapping by dams under foreseeable future scenarios are expected to have already reached or to reach the VMD within the next 10–20 years [ MRC, 2011 ]. The effect of sediment trapping on the the coarse sediment (sand and gravel), which is a very small fraction of over-all sediment load, takes a long time to travel downstream, but the effect of fine sediment trapping can almost immediately travel to the Delta. Furthermore, although sand export is banned in Cambodia and Viet Nam, domestic consumption has persisted. Projections to 2040 [ SIWRP, 2015 ] show over 1,500 M m3 demand within the VMD for infrastructure development. Current sand mining within the VMD and upstream in Cambodia, are estimated at 28 M m3/yr (40–50 Mt yr-1) [ Bravard et al., 2013; Eslami et al., 2019b; Jordan et al., 2019 ]. “Considering that this does not account for sand mining in Cambodia, and possible illegal sand mining, only in Viet Nam, this amount

is likely to be close to 100% (or more) of the total fluvial sediment supply” [ Eslami et al., 2019b ]. With this starvation exceeding natural replenishment, it leads to erosion of the capital sediment (older Holocene fossil deposits), and explains the incision of the riverbeds [ Brunier et al., 2014 ].

Rapid erosion of riverbed levels [ Doan et al., 2018; Eslami et al., 2019b ] has already resulted in significant tidal amplification and increased saline water intrusions within the VMD [ Chapter 9 ]. The observed increasing trends for saline water intrusions along the estuarine channels — in the range of 0.2–0.5 PSU/year – is directly linked to sediment deprivation and riverbed erosion [ Eslami et al., 2019b ]. Furthermore, although Mekong River dry season discharge has increased, total dry season discharge to the Delta has not changed significantly. Therefore, with deeper riverbeds and the relative sea level rise, the delta is far more vulnerable to saline water intrusions [ Eslami et al., 2021a ]. Because of this combination, actual salinity intrusions in 2016 and 2020 already surpassed [ IFRC, 2020 ] the 2050 projections, developed in 2015 [ Smajgl et al., 2015 ].

4.2 Surface and groundwater resources

The VMD is highly populated, and the riparian countries in the Lower Mekong Basin are developing with a focus on economic growth. For growing economies, water is an important natural resource that fuels development. Rapid economic development leads to increased pressure on water availability. Water resources in the VMD are facing several challenges, which can be divided into external or internal origins. The external factors influencing water resources in the VMD include the rapid development of hydraulic structures in the upstream countries, projected climate change and global sea level rise. The upstream countries have built and planned hydropower dams and irrigation projects on the mainstream and tributaries of the Mekong River. Due to the downstream location, any change in the upstream influences the VMD through changes in the flow regime and sedimentation [ Lauri et al., 2012; Van et al., 2012; Manh et al., 2015 ].

The internal factors consist of dykes, polders, infrastructure to prevent flooding and salt intrusion, and high water demand. The intensification of agricultural and aquacultural crops leads to increased freshwater use, and hence pressure on surface water resources from the Mekong river. Therefore, the local inhabitants look for other water resources and, as a result, groundwater extraction has been growing rapidly over the past three decades, from very limited amounts in the 1990’s to more than 2.5 106 m3/day nowadays [ Minderhoud et al., 2017 ]. The water is extracted from different aquifers (i.e. sand bodies) in the subsurface, at depths that range in some places to more than 500m. The water pressure (i.e. hydraulic head) in the aquifers has been monitored since 1990, when the groundwater situation was still rather undisturbed in many places. Since then, following the steady increase in extractions, the water pressure in the different aquifers has shown decreasing trends throughout the delta [ Figure 7.15 ]. Following the drop in pressure, water is drained from fine-grained deposits (i.e. aquitards, low permeable layers separating the different aquifers), causing compaction of the aquifer-system and consequent land subsidence. This anthropogenically-driven compaction is currently the main driver of accelerating land subsidence in the VMD [ Erban

[ Figure 7.14 ] Groundwater extraction in the VMD and impact on the aquifers

a) Estimated groundwater extraction in the VMD in million m3 per day showing a strong and consistent increase since the 1990s (modified after Minderhoud et al., 2020b). b) Cross-sections of the multi-aquifer system, i.e. aquifers and aquitards, of the VMD from which groundwater is extracted at different depths (Modified after Minderhoud et al., 2017) . c) Hydraulic head time series measured for different aquifers in Soc Trang. All confined aquifers show consistent declines in hydraulic head driven by groundwater extraction. The declining trends are measured in aquifers throughout the VMD (Data by DWRPIS).